1. Field of the Invention
The present invention generally concerns reverse osmosis water purification systems, and especially the housing for an reverse osmosis membrane used in a reverse osmosis water purification system.
The present invention particularly concerns the design of an all-plastic housing for a reverse osmosis membrane used in a reverse osmosis water purification system, and a method of constructing such a housing, so that, nonetheless that the constructed housing is easy and quick to assemble about an enclosed membrane by hand and totally without tools, the housing exhibits great strength and reliability.
2. Description of the Prior Art
2.1 General Construction of Housings for Reverse Osmosis Membranes, or Membrane Housings
Housings for reverse osmosis membranes, or membrane housings, are commonly made of various combinations of three materials. Each material used must be essentially inert to, and uncorrupted or corroded by, the liquids with which it comes into contact during the reverse osmosis process. Because the liquids at the exterior of the membrane within the housing are, in accordance with the principles of reverse osmosis, necessarily at a considerable pressure, the material of the housing must have considerable structural strength.
Stainless steel is commonly used for the highest pressure applications, typically for pressures in the range over eight hundred pounds per square inch (&gt;800 lbs./inch.sup.2), and typically less than one thousand pounds per square inch (&lt;1000 lbs./inch.sup.2) Filament wound fiberglass may also be used for the same high pressures. Finally, thermoplastic, and most commonly polyvinyl chloride, or PVC, is used for low osmotic pressures up to approximately two hundred pounds per square inch (&lt;200 lbs./inch.sup.2).
An osmosis membrane is commonly shaped as an elongate cylinder, and so also is the shape of the housing holding the cylindrical membrane. Low pressure membranes are commonly available in sizes 4".times.14", 4".times.21", 4".times.40", and 2 @ 4".times.40" where 4" is the outside diameter and the 14", 21", or 40" is the length. The inside diameter, or bore diameter, of the osmosis membrane is commonly about one-half inch (1/2"), terminating at both ends of the bore in a short, three-quarter inch (3/4") pipe stub. The principle of the use of the osmotic membrane is that an unpurified and pressurized inlet fluid is fed into the membrane along its one end while a concentrate fluid is withdrawn from the exterior surface of the membrane at the opposite end. Such portion of the pressurized fluid as penetrates through the membrane to its central bore exhibits a greatly reduced pressure, and is drawn off from either, or both, ends of this central bore as the purified, or product, fluid.
The housing that fits about the osmotic membrane accordingly has, for a one common type of membranes, an interior diameter of four inches (4"). The housing is of a length suits the enclosure of the full length of the membranes, which are commonly of lengths 14", 21" or 40". A housing will sometimes enclose two membranes such as, for example, 2 @ 4".times.40".
2.1 Problems With the Construction and Use of Membrane Housings
Membrane housings must have removable end sections, or caps, in order that access may be obtained to the contained filter for filter replacement. The opening to the filter must be of a diameter as great as is the filter itself, or commonly 4". When the housing and its contained filter are in use in a reverse osmosis system, the fluid inside the housing is pressurized, commonly at up to two hundred and thirty pounds per square inch (230 P.S.I.). The force exerted by this pressure against a circular plug of four inch outside diameter (4" O.D.) and approximately twenty-five square inches (25 in.sup.2) area is on the order of five thousand seven hundred and fifty pounds (5,750 lbs.), or two and seven-eighths tons (2.875 tons) of force.
One common construction of and end plug, or cap, is applied to a reverse osmosis membrane housing that has, as its central structural element, a thick and robust custom-extrusion cylindrical pipe, commonly of PVC. The PVC pipe is commonly of four inch internal diameter (4" I.D.) with a twenty-six hundredths inch (0.26") wall thickness. A circular plug of four inches outside diameter (4" O.D.) and a typical thickness of one inch (1") is retained within the pipe at a position typically one and one-half inches (11/2") from the end of the pipe. The circular plug is typically so maintained by removable pins or dowels, typically two in number made from quarter inch (1/4") stainless steel rod, that extend through opposing (1/4") holes in the wall of the pipe and transversely across the (4") bore of the pipe. The two transverse pins are typically spaced parallel at a separation from each other of two and one-quarter inches (2 1/4"), and are spaced each one at about one-half inch (1/2") maximum separation from that interior sidewall of the pipe to which it is most closely adjacent.
The one inch (1") thick circular plug presents circumferential channels or grooves--typically two in number--in which are typically placed neoprene rubber "O"' rings to serve as sealing gaskets. Fluid flow access to the filter within the pipe, and the housing, is through ports in the cap. A first port in the cap is typically located midway between the center and the rim, and permits fluid flow access to the filter for purposes (as the connection dictates) of either (i) unpurified input or (ii) concentrate output. An optional second port in the cap is centrally located and serves, when opened, to permit flow communication with the axial bore of the filter for the purpose of retrieving the purified, product, output fluid.
There are several problems presented with this construction. This first, and most dire, problem is that the entire assembly is prone to catastrophic failure in use, hazarding the severe flooding of the premises in which a reverse osmosis system is installed. Because the four holes in the sidewall of the PVC pipe into which the stainless steel pins are inserted serve to weaken the pipe at this location, an exterior surround band, also typically of stainless steel, is use to surround the pipe and to also engage the ends of the stainless steel retaining pins. No substantial redistribution of the high local specific forces at the four pin holes may be accomplished unless, and until, the surround band is affixed to the circumference of the pipe. The surround band is so typically affixed by glue and/or by its complimentary fit into a shallow exterior circumferential groove to the pipe. Because the location of the (i) four pin holes, or (ii) the shallow exterior circumferential groove, are the structurally weakest points of the PVC pipe, the pipe typically fails by completely rupturing into two separate pieces at a one of these locations.
Another problem is presented with accessibility to the filter. The stainless steel pins are prone to contamination, and must typically be driven from their seated positions (holding in the end plug, and filter) by use of a hammer and drift punch, often in tight quarters. Reinsertion of the stainless steel retaining pins is equally cumbersome, and normally requires effort to align the pins into their transverse holes, and hammering.
Still another problem is presented with the location of the "O" ring seals to the rim of the circular plug. Although the pressure against these rings permits fluid-tight sealing, it is clear that the "O"-rings are, as seated within their invariant channels or grooves at the rim of the plug, not in compression between any two complimentary surfaces, but only as between one surface (a side of the channel, or groove, at the rim of the plug) an the pressurized fluid itself. This is an awkward use of a seal, or gasket, best and most reliably used in compression between two solid surfaces of complimentary contour. This awkwardness may be why manufacturers of prior art osmosis membrane housings often advertise and promote "double seals", meaning two "O" rings per sealed channel per plug. Logically, if a seal was properly deployed then one such should suffice. (The present invention will prove to have redundant, dual, seals in one location only, which location is not equivalent to the location now discussed. The use of redundant, or dual, seals in the preferred embodiment of the present invention may considered to be as much for market acceptance, and user emotional comfort, as for any practical utility or necessity.)
A third problem is presented with the location of the flow connections at the plug which is seated well below the end of the pipe. It will be recalled that there is an inlet flow connection at one end of the housing for channeling unpurified, inlet, fluid into contact with the exterior surface of the osmosis membrane at a one end thereof. Likewise, there is a first outlet flow connection at the other end of the housing for channeling the waste, concentrate, fluid away from the osmosis membrane at its other end. Finally, there is a second outlet flow connection at one, or at both, ends of the membrane's central bore by which connection(s) the purified, product, fluid is extracted. In the prior art all these flow connections are through the plugs at each end. Logically, and in actual real-world use, less cumbersome movement of plumbing connecting to the housing would be required if at least some of the flow connections were to, and through, the cylindrical sidewalls of the housing as opposed to being through its end plugs.
The present invention will be seen to be generally concerned with an entire re-engineering of a reverse osmosis membrane housing so that (i) an housing constructed entirely of plastic is (ii) easy and quick to assemble about an enclosed membrane by hand and totally without tools, while (iii) exhibiting great strength and reliability. The (iv) locations at which the external, plumbed, fluid flow lines are connected to the housing are also slightly changed, generally improving thereby ease of access to the housing and its contained osmosis filter.